The invention described herein relates to nuclear fuel and more particularly to an improved design of nuclear fuel pellet which densifies and increases in size at substantially the same rates while undergoing a fission process in a nuclear reactor.
The typical fuel rod used in commercial nuclear reactors measures about 8 to 14 feet in length and contains multiple fuel pellets, each being about 0.60 inch long by 0.366 inch diameter. As manufactured, these pellets are porous and the void spaces therein are called pores. During reactor operation, the fuel pellets are irradiated and produce fission products which cause the pellets to swell. In some cases, such swelling can place strains sufficiently great on the fuel rod walls as to cause the fuel rod to fracture or fail and release radioactive particles to the reactor coolant. On the other hand, the fuel pellets may also densify as a result of dispersion of individual pores by the fission process, and as a result of the migration of the products of dispersion (vacancies) to pellet boundaries. This action creates voids in the fuel pellet column that may cause reactivity changes, heat transfer problems or radial collapse of the fuel rod, all of which will adversely affect reactor operation. This invention deals with the steps which may be taken to eliminate or substantially reduce fuel pellet swelling and densification by balancing one against the other. The circumstances which give rise to the densification-swelling problem show that the problem can be eliminated by taking special care during the fuel pellet manufacturing operation.
When ceramic nuclear fuel pellets are prepared by pressing and sintering of powders, the product invariably contains more or less residual porosity. Little effort has been directed toward removing this residual porosity because of a wide-spread belief in the industry that porosity in nuclear fuel pellets is somehow useful in retarding the swelling that results from the accumulation of the new, extra atoms that are produced by the fission of one uranium atom into two fission product atoms. These extra atoms almost invariably come to rest within the body of the nuclear fuel and increase its volume slowly and inexorably.
In the absence of any real evidence, it has been assumed that all porosity in a ceramic nuclear fuel pellet is used to reduce swelling, and that no net swelling is possible until all the porosity is consumed. This belief is evident in the calculations of fuel swelling made by others working in the art. It also has been generally assumed that the replacement of pore volume by fission product volume takes place by some unknown mechanism on a one for one basis. As a logical consequence, it has been assumed that all that is necessary to increase the burnup, i.e., the number of fission events that can be accumulated before the fuel pellet begins to swell and thus increase its outer dimensions, is to increase its initial porosity. This widespread and general belief led to fuel pellet designs of lower and lower density until recently when it was discovered that low density fuel pellets were densifying inreactor at low burnups and at low temperatures. Studies show that sufficient porosity is removed early in the irradiation history of a low density fuel pellet to increase its density. Such behavior shows that the replacement of pore volume by swelling does not take place on a one for one basis and, unless a reasonable balance exists between pore volume and swelling, swelling will predominate later in the life of the fuel and cause fuel rods to fail structurally and detrimentally affect reactor operation. These studies further led to the realization that pores do not interact mechanically, directly and instantaneously with fission product atoms. Any interaction that takes place must be the result of a complex sequence of events. Such considerations led to more extensive studies of the processes of pore removal, and of swelling, as two separate and distinct processes, and these concepts form the basis of this disclosure.